SIDEBAR 2.1Nuclear Medicine Imaging
This sidebar describes the nuclear medicine imaging methods (modalities) that are in common use today in the United States and many other countries.
Single Photon Emission Computed Tomography (SPECT) generates three-dimensional (3D) images of tissues and organs using radionuclides that emit gamma rays; the most commonly used radionuclide is Tc-99m. Individual gamma rays emitted from the decay of these radionuclides (i.e., single photon emissions) are detected using a gamma camera. This camera technology is used to obtain two-dimensional (2D) images; 3D SPECT images are computer generated from a large number of 2D images recorded at different angles.
Positron Emission Tomography (PET) generates 3D images of tissues and organs using tracers that emit positrons (i.e., positive electrons): for example, fluorine-18 (F-18). Annihilation reactions between the positrons from these radionuclides and electrons present in tissues and organs produce photons. (Two photons are emitted simultaneously for each annihilation reaction and essentially travel in opposite directions.) The photon pairs are detected with a camera having a ring of very fast detectors and electronics.
PET images generally have a higher contrast and spatial resolution than do SPECT images. However, PET equipment is more expensive and therefore not as widely available as SPECT equipment. Additionally, most PET tracers have short half-lives (e.g., nitrogen-13 (N-13): 10 minutes, carbon-11 (C-11): 20 minutes, and F-18: 110 minutes), so they have to be produced close to their point of use.
Both SPECT and PET cameras have been paired with CT to provide functional and anatomic imaging (SPECT/CT or PET/CT; see ). PET cameras have also been combined with MRI (PET/MRI). These hybrid images improve diagnostic accuracy.
(A) planar (2D) whole body Tc-99m MDP bone scan taken from the front (ant) and back (post) combined with (B) SPECT/CT images used for disease diagnosis.
